Concealed search is a technique that can perform search with encrypted data and keywords being maintained in an encrypted form.
In recent years, popularization of cloud services and the like has enabled data management on the internet. Data management on the internet, however, includes a risk of data leakage to the outside because a cloud server where data is kept may be infected by malware such as a computer virus and that a cloud service administrator may commit fraudulence. Encryption technology is available as a way of avoiding such a security threat, but this technology poses a problem that if data is simply encrypted, the data cannot be searched for. To deal with this problem, a scheme has conventionally been proposed according to which data is searched for after the data is temporarily decrypted on the cloud server. However, since the data is restored to and maintained as a plaintext in the cloud server for a predetermined period, this scheme may be insufficient as a countermeasure. Today, a variety of studies on a concealed search technique being an encryption technique are underway to enable search of data in the encrypted form. Many specific schemes as the concealed search technique have been disclosed in recent years.
The concealed search technique roughly consists of two types: a scheme that utilizes common key encryption (see, for example, Patent Literature 1, Patent Literature 2, Non-Patent Literature 1, Non-Patent Literature 2, and Non-Patent Literature 3); and a scheme that utilizes public key encryption (see, for example, Non-Patent Literature 4 and Non-Patent Literature 5). The common key encryption is also called secret key encryption or symmetric key encryption.
With a public key encryption-base concealed search technique, any entity can register data with a data center by using a public key. An entity keeping a secret key that corresponds to this public key can perform keyword search.
On the other hand, with a common key encryption-base concealed search technique, only an entity keeping a secret key can execute data registration and keyword search. It is known that with the public key encryption-base concealed search technique, a search process is performed at a higher speed compared to the existing public key encryption-base secret search technique. In particular, recent common key encryption-base secret search technique has achieved speed-up by using a data structure in which a search result called an index is stored in an encrypted form.